mgmt/osdp: Fix osdp_build_ctx(); Add NULL check on ctx
mgmt/osdp: Fix osdp_build_ctx(); Add NULL check on ctx

osdp_build_ctx() was returning fixed NULL and the subsequent code did
not check if ctx was NULL due to osdp_build_ctx() failures. Fix both
these issues.

Signed-off-by: Siddharth Chandrasekaran <cf3591b207d7ad6b41008948dad8c0c751800981@embedjournal.com>

Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr